Chlorine dioxide (ClO2) is a selective oxidizing agent widely used in pulp bleaching, water disinfection and numerous other applications. Due to its inherent instability, it cannot be transported and, therefore, is produced in situ at its point of use.
Commercial methods for chlorine dioxide generation are based on two types of precursor chemicals, namely chloric acid/chlorates and chlorous acid/chlorites.
Large-scale ClO2 generators, typically used in pulp bleaching applications, are usually based on the reduction of acidified chlorate ion solution, whereas smaller scale applications, such as water treatment and disinfection, utilize a one-electron oxidation of chlorite ion, employing a wide variety of oxidizing agents, such as chlorine, hypochlorite, chlorous acid, persulfate, etc.
The most commonly used oxidizing agent utilized in the latter process is chlorine which may be the form of gas or in solution. The chlorine dioxide generation reaction proceeds in solution according to the following equations (1), (2) and (3) and overall equation (4):Cl2+H2O→HOCl+H++Cl− (pH<7)  (1)                [Hypochlorous acid]2ClO2+2H+→2HClO2 (pH<8.3)  (2)        [Chlorous acid]2HClO2−+HOCl→2ClO2+Cl−+H2O+H+  (3)        [Chlorine dioxide]2ClO2−+Cl2→2ClO2+2Cl−  (4)An undesirable reaction occurs at higher pH with excess hypochlorous acid, namely:HOCl+2ClO2→ClO3−+Cl−+H+  (5)        
In order to ensure a high conversion of chlorite ion to chlorine dioxide, an excess of chlorine is required, which is added first to water to reduce the pH of the resulting aqueous medium to less than 7. In practice, this excess of chlorine can range from about 5 to about 25% excess over stoichiometric requirements, for production of chlorine dioxide according to equation (4). This excess, as with all chemical reactors of this type, is dependent upon the degree of mixing and residence time within the reaction zone, which is typically only a fraction of a second, and the concentration of the feed reactants. However, the excess chlorine can react with the product chlorine dioxide in accordance with equation (5), reducing the overall yield. Excess chlorine can also form chlorinated disinfection by-products (DBP's), depending upon the organic content of the water. Reactions according to equations 1 to 4 are only dependant upon the degree of mixing. Typically, the pH of commercial sodium chlorite solutions is between about 9 to about 12, and this must be neutralized before reaction according to equation (2) can proceed, which is achieved by adding chlorine first.
There are numerous commercial chlorite-based ClO2 generators available on the market which can satisfy these conditions. In a conventional ClO2 generator, chlorine gas is mixed with water to produce hypochlorous acid, which then is mixed with alkali metal chlorite in a reaction chamber. This second reactant, (i.e. alkali metal chlorite), can be introduced to the reaction chamber either by pumping or induced by a vacuum device, such as a water eductor, which serves also to absorb the product chlorine dioxide in solution. Operating under vacuum in this manner is much preferred owing to its simplicity, and allows the use of concentrated sodium chlorite solution (typically about 25% w/w) and pure chlorine gas fed under vacuum directly into the device, thus vastly aiding reaction kinetics. However, water eductors are single volumetric capacity devices which are set by the water pressure provided, and the size selected. Thus, if the feed volumes of the reactants is reduced, then the vacuum exerted increases, which, in turn, reduces the reaction time available, because the two-phase reaction mixture is mainly gas.
Numerous patents related to the above described subject matter claim features, such as the order in which precursors are added, the relative positions of water ejectors/chemical feed pumps, the mode of operation (continuous vs intermittent), etc. A detailed description of chlorite ion based chlorine dioxide generators available on the market as of 1998 is described on pages 23 to 54 of D. Gates' book “The Chlorine Dioxide Handbook”, Chapter 3 (“Commercial Designs for Full-Scale Chlorine Dioxide Generators”), the disclosure of which is incorporated herein by reference (along with all the patents and publications cited therein.).
All of the water eductor generators described in the above-mentioned reference are designed in such a way as to exhibit an optimum performance at a fixed production rate, specific to a fixed size eductor. In some cases, this can be compensated for by varying the addition of water to the reaction zone, but this only partially alleviates the change in conditions, as it reduces the concentration of the reactants in the reaction zone as well. The required output of a typical Municipal Water Treatment facility varies substantially during day and night (typically by a factor of two), and also seasonally between summer and winter (typically also a factor of two). In order to compensate for the reduction in efficiency experienced with current devices, users need to either switch to a second or third generator, sized to accommodate the changed capacity, or shut down and install a smaller or larger eductor.
The utilization of high feed concentrations of sodium chlorite has previously been described to be beneficial. Typically, about 25% w/w sodium chlorite solution is used in present practice. Higher concentrations with existing devices can lead to pluggage and scaling problems.
The use of a less concentrated sodium chlorite feed solution has a significant, negative impact on the overall process economics, due to increased chlorite storage requirements and concentration costs, as well as the equipment used in some cases to prepare weaker solution on-site prior to use.
There is a need, therefore, to develop a simple, yet reliable chlorine dioxide generating system which can operate efficiently over a wide range of capacities, with a minimum excess chlorine, and at the same time able to accept a more concentrated alkali metal chlorite feed solution, typically in the range of up to at least about 31 wt. % and preferably up to about 38 wt. %.
One recent proposal to alleviate the prior art problem of variable production rate is described in U.S. Pat. No. 5,968,454. However, this approach is deficient owning to its complexity, lack of reliability and inability to accept a concentrated alkali metal chlorite feedstock solution.